netdev - Re: [PATCH bpf] bpf: avoid false sharing of map refcount with max

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Message-ID: <51893658-b798-c8da-fc9f-3e65f771eaec@solarflare.com>
Date:   Tue, 9 Jan 2018 16:23:08 +0000
From:   Edward Cree <ecree@...arflare.com>
To:     Daniel Borkmann <daniel@...earbox.net>, <ast@...com>
CC:     <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf] bpf: avoid false sharing of map refcount with
 max_entries

On 09/01/18 12:17, Daniel Borkmann wrote:
> In addition to commit b2157399cc98 ("bpf: prevent out-of-bounds
> speculation") also change the layout of struct bpf_map such that
> false sharing of fast-path members like max_entries is avoided
> when the maps reference counter is altered. Therefore enforce
> them to be placed into separate cachelines.
>
> pahole dump after change:
>
>   struct bpf_map {
>         const struct bpf_map_ops  * ops;                 /*     0     8 */
>         struct bpf_map *           inner_map_meta;       /*     8     8 */
>         void *                     security;             /*    16     8 */
>         enum bpf_map_type          map_type;             /*    24     4 */
>         u32                        key_size;             /*    28     4 */
>         u32                        value_size;           /*    32     4 */
>         u32                        max_entries;          /*    36     4 */
>         u32                        map_flags;            /*    40     4 */
>         u32                        pages;                /*    44     4 */
>         u32                        id;                   /*    48     4 */
>         int                        numa_node;            /*    52     4 */
>         bool                       unpriv_array;         /*    56     1 */
>
>         /* XXX 7 bytes hole, try to pack */
>
>         /* --- cacheline 1 boundary (64 bytes) --- */
>         struct user_struct *       user;                 /*    64     8 */
>         atomic_t                   refcnt;               /*    72     4 */
>         atomic_t                   usercnt;              /*    76     4 */
>         struct work_struct         work;                 /*    80    32 */
>         char                       name[16];             /*   112    16 */
>         /* --- cacheline 2 boundary (128 bytes) --- */
>
>         /* size: 128, cachelines: 2, members: 17 */
>         /* sum members: 121, holes: 1, sum holes: 7 */
>   };
>
> Now all entries in the first cacheline are read only throughout
> the life time of the map, set up once during map creation. Overall
> struct size and number of cachelines doesn't change from the
> reordering. struct bpf_map is usually first member and embedded
> in map structs in specific map implementations, so also avoid those
> members to sit at the end where it could potentially share the
> cacheline with first map values e.g. in the array since remote
> CPUs could trigger map updates just as well for those (easily
> dirtying members like max_entries intentionally as well) while
> having subsequent values in cache.
>
> Quoting from Goolge's Project Zero blog [1]:
typo "Goolge".